Membrane fusion must usually accompany biological processes such as secretion, hormone and enzyme release, intracellular transport and cell growth. The question of reorganization that must accompany fusion has been an important one but an intractable problem in membrane biology. Our results on membrane fusion during mucocyst secretion in Tetrahymena pyriformis have opened up an entirely new dimension in the study of membranes and led to our hypothesis of membrane fusion (B. Satir et al., 1972, 1973). We have demonstrated specific interior membrane markers -- rosettes -- within the plasma membrane and corresponding ones -- annulus -- within the membrane of the secretory vesicle. These arrays are formed via membrane rearrangement prior to fusion and discharge. Further, we have convincing evidence (B. Satir, 1974) that after discharge the vesicular membrane becomes incorporated into the plasma membrane, thus demonstrating bulk membrane growth. Synchronous discharge of mature mucocysts have been induced using the dibucaine method of Thompson et al., 1974, which event is followed by total deciliation. The deciliated cells recover when returned to fresh medium and metabolic studies of mucocysts and ciliary regrowth will be done. Isolated membranes of ciliary and mucocyst membranes will be analyzed biochemically to elucidate if the two perhaps are related, one acting as precursor to the other (B. Satir et al., 1975).